ltc1760 Linear Technology Corporation, ltc1760 Datasheet - Page 30

• Manufacturer Part Number: ltc1760
• Description: Dual Smart Battery System Manager
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC1760
at hand. The average current control loop converts the
voltage between CSP and BAT to a representative current.
Error amp CA2 compares this current against the desired
current programmed by the I
I
The voltage at BAT is divided down by an internal resistor
divider set by the V
decrease I
reference.
The amplifier CL1 monitors and limits the input current,
normally from the AC adapter, to a preset level (100 mV/
R
voltage and thus reduce battery charging current.
An over-voltage comparator, OV, guards against transient
overshoots (>7.5%). In this case, the top MOSFET is
turned off until the over-voltage condition is cleared. This
feature is useful for batteries which “load dump” them-
selves by opening their protection switch to perform
functions such as calibration or pulse-mode charging.
The top MOSFET driver is powered from a floating boot-
strap capacitor C4. This capacitor is normally recharged
from V
is turned off. As V
battery voltage, the converter will attempt to turn on the
top MOSFET continuously (“dropout’’). A dropout timer
detects this condition and forces the top MOSFET to turn
off, and the bottom MOSFET on, for about 200ns at 40µs
intervals to recharge the bootstrap capacitor.
7.1 Charge MUX Switches
The equivalent circuit of a charge MUX switch driver is
shown in Figure 5. If the charger controller is not enabled,
the charge MUX drivers will drive the gate and source of
BGATE
30
TGATE
INDUCTOR
TH
CURRENT
CL
for the desired voltage across R
). At input current limit, CL1 will decrease the I
OFF
OFF
ON
ON
CC
through an external diode when the top MOSFET
TH
if the divider voltage is above the 0.8V
U
DAC
IN
decreases towards the selected
and is used by error amp EA to
Figure 4.
DAC
t
OFF
at the I
SENSE
TRIP POINT SET BY I
SET
.
pin and adjusts
TH
VOLTAGE
TH
CHARGER
the series connected MOSFETs to a low voltage and the
switch is off. When the charger controller is on, the charge
MUX driver will keep the MOSFETs off until the voltage at
CSN rises at least 35mV above the battery voltage. GCH1
is then driven with an error amplifier EAC until the voltage
between BAT1 and CSN satisfies the error amplifier or until
GCH1 is clamped by the internal Zener diode. The time
required to close the switch could be quite long (many ms)
due to the small currents output by the error amp and
depending upon the size of the MOSFET switch.
If the voltage at CSN decreases below V
comparator CC quickly turns off the MOSFETs to prevent
reverse current from flowing in the switches. In essence,
this system performs as a low forward voltage diode.
Operation is identical for BAT2.
7.2 Dual Charging
Note that the charge MUX switch drivers will operate
together to allow both batteries to be charged simulta-
neously. If both charge MUX switch drivers are enabled,
only the battery with the lowest voltage will be charged
until its voltage rises to equal the higher voltage battery.
The charge current will then share between the batteries
according to the capacity of each battery.
When batteries are controlled charging, only batteries
with voltages above V
a battery is wake-up charging this restriction does not
apply.
BATTERY
FROM
TO
Figure 5. Charge MUX Switch Driver Equivalent Circuit
1
BAT1
CSN
35mV
20mV
(CHARGE PUMPED)
–
+
+
–
DCIN + 10V
CHMIN
CC
EAC
OFF
are allowed to charge. When
10k
BAT1
GCH1
SCH1
– 20mV a
Q4
Q3
1760f
1760 F05